1. Field of the Invention
The present invention generally relates to compositions of and methods used for hydrocarbon exploitation such as the drilling and production of wells, especially oil and gas wells. More particularly, the invention relates to such compositions and methods, which alter the physical or chemical properties of a fluid system, including additives for controlling fluid losses during hydrocarbon exploitation processes.
2. Description of Related Art
In the drilling industry, a hollow drill pipe with a rotary bit attached to the lower end is typically rotated in a borehole to make an oil, gas, thermal or water well. A fluid system (e.g. drilling fluid or “mud”) is pumped down the hollow drill pipe, through the bit at the bottom of the borehole, and then up to the surface through the annular space between the drill pipe and the borehole wall. The fluid system lubricates and cools the rotating drill bit, suspends and carries cuttings out of the borehole, coats the wall of the hole with a thin impervious layer of solid material to prevent flow of fluids into or out of the formation, and also exerts a hydrostatic pressure on the formation to counterbalance the pressure of liquids or gases present therein. Loss of fluid can occur due to porosity or cracks in the formation.
Typically, the fluid system is primarily a suspension of solid material, such as clay, in a liquid such as water, but it may also contain a variety of additives or improvements. For the fluid system to perform its functions, it must have certain physical properties. The fluid must have viscosity, and yet be readily pumpable. It must be sufficiently thixotropic to suspend the cuttings in the borehole when fluid circulation stops. The fluid must release cuttings from the suspension when agitating in the settling pits. It must form a thin impervious filter cake on the borehole wall to prevent loss of liquid from the fluid system by filtration into the formations.
A filter cake forms when the fluid system contains particles of size only slightly smaller than the size of the pore openings of the formation. Such a filter cake effectively seals the borehole wall to inhibit any tendencies of sloughing, heaving or cave-in of rock into the borehole. The liquid, which enters the formation while the cake is being established, is known as the surge loss or spurt loss, while the liquid, which enters after the cake is formed, is known as the filtrate. Both the spurt loss and filtration rate must be minimized when penetrating potentially productive formations in order to minimize any damaging effects from fluids entering the formation.
Further, the fluid must be capable of suspending high specific gravity weighting agents, such as barite or other inorganic heavy metal compounds, to maintain sufficient pressure against a formation when necessary. The fluid system should also be able to assimilate finely divided drill cuttings formed during hydrocarbon exploitation operations, and should be able to accept some chemical contaminates without excessively disrupting the physical properties of the fluid system.
To establish and maintain the desired physical properties, a variety of chemicals, clays and weight materials are added to water-based fluids. Rock particles and low-yielding clays are generally incorporated into the fluid system to provide viscosity to the fluid system, to deposit a filter cake that will seal permeable formations in order to limit filtration losses and prevent stuck pipe, and to provide buoyancy for drill cuttings. These solids also affect many fluid system properties adversely, however. As mentioned above, formation clays are unavoidably incorporated into the fluid system, and depending on their nature and amount, the clay minerals can be beneficial or harmful to the fluid system. Because it is not possible to remove all drill solids, especially the very small, colloidal particles, either mechanically or by other means, they must be considered a continual contaminant of a fluid system. Contaminants, such as gypsum, can “cut” the fluid system causing particles to flocculate and the viscosity to increase. When this occurs, there is danger of torquing the drill pipe to the point of breakage or of causing a blowout. At high temperatures, gelation or thickening of the fluid can occur, leading to greatly increased pressure on the recirculation pump. A balance of all the favorable and adverse effects must be achieved in order for the fluid system to provide pressure control to prevent an influx of formation fluid, provide energy at the bit to maximize rate of penetration, provide wellbore stability through pressured or mechanically stressed zones, suspend cuttings and weight material during static periods, permit separation of drilled solids and gas at the surface, and to remove cuttings from the well.
Generally, the term “clay” is used to describe premium ground clay minerals, such as Wyoming bentonite, that are added to increase fluid viscosity and to improve filter cake. Drill cuttings, barite and other solids, however, will increase viscosity (resistance to flow), especially if the particle size degrades into the colloidal range. Colloidal solids produce most of the viscosity in fluid systems due to this surface area increase. For that reason, the volume of colloidal-size solids contained in fluid systems must be controlled and the cation exchange capacity, water adsorption and surface area of clay particles must be taken into account in order to minimize hydrocarbon exploitation problems. Not only is the surface area of clay particles important in determining a fluid's resistance to flow, the clay's chemical composition and its “activity” or electrical charge characteristics also affect how water and chemical contaminants or treatments will interact with the clay particles to alter the fluid's properties.
In an attempt to avoid or to compensate for certain effects, a variety of fluid system additives have been used in the drilling industry. Additives that reduce the spurt loss and filtration rate are referred to as fluid loss control agents. Additives that reduce flow resistance and gel development in fluid systems are referred to as thinners or deflocculants. Some of these additive materials include starches (e.g. corn, potato), starch derivatives, water soluble cellulose derivatives, humates, plant tannins, polyphosphates or phosphate-containing materials, lignite materials, lignosulfonates and synthetic polymers. One of the problems with some of those materials is that they are unstable at the higher temperatures that are typically encountered downhole. Cost and environmental effects of the additives are also important factors. What is needed is a commercially attractive multifunctional fluid system additive, such as a fluid loss control additive, that is capable of stabilizing the fluid properties over a range of temperatures and contaminant levels under hydrocarbon exploitation conditions.